In a typical home networking scenario, both high spatial coverage (e.g., coverage to various parts of the home) and high temporal coverage (e.g., coverage nearly 100% of the time) are sought. Wireless communications often experience variations in the temporal coverage, which may be due to interference (in-band or out-of-band) and/or multipath characteristics. Multiple input-multiple output (MIMO) techniques are sometimes used to increase the amount of information that can be communicated within a predetermined time by creating multiple spatial channels through the use of beamforming techniques, e.g., in the IEEE 802.11n standard. In a MIMO system, beamforming, or pointing the reception in a certain direction (or transmission in a transmission system), is accomplished in a similar manner as with a dish antenna. A continuous surface is approximated with a number of smaller antennas. The smaller antennas are nearly collocated and fed with signals of certain phase relationships. The antenna separation is typically on the order of one quarter wavelength at the frequency of operation, e.g., a few inches apart from one another at a WiFi frequency of 5.8 GHz. The various antennas exploit spatial diversity, e.g., using different receive angles. An N×N matrix relating each transmit antenna to each receive antenna is provided in MIMO systems and is well known to one of ordinary skill in the art. If multipath allows for N different independent or uncorrelated paths between a transmitter and a receiver, then N antennas can sustain a maximum of N spatial channels. In such a system, each receive antenna can receive a linear combination of N bitstreams. If the channels are partially correlated, the number of available spatial channels drops. For example, with 50% correlation, half the spatial channels are unavailable. Beamforming is a technique used in MIMO antenna arrays for directional signal transmission or reception, which may increase the link margin and improve the coverage and range. Spatial selectivity is achieved by using adaptive or fixed receive/transmit beam patterns. High spatial selectivity through beamforming is achieved with a large number of antennas located close to one another. The antenna resources can be used to increase the capacity by generating several spatial channels. Alternatively, the antenna resources can be used to increase the coverage by improving the link budget with beamforming. As the number of spatial channels increases, the coverage decreases due to a reduction in the link budget. Accordingly, it can be seen that there is a trade-off between the number of spatial channels and the extent of the coverage.
Current MIMO communication techniques suitable for home networking are limited to up to 4 transmit antennas and 4 receive antennas. This configuration does not provide enough throughput for reliable, high-performance networking in a home environment. For example, future throughput needs are likely to be on the order of 100 Mbps or more. A 4×4 MIMO having two spatial channels and beamforming can provide 100% spatial coverage (i.e. coverage in any place in the home) with a throughput of about 20-30 Mbps. However, less than 90% coverage is typical for a throughput of 50 Mbps or more. Accordingly, in order to get reasonable coverage with at least 100 Mbps, more than four receive antennas are needed (e.g., eight or more receive antennas).
In known communication systems involving multiple receiving antennas, each antenna has a dedicated receiver and analog to digital converter (ADC). In the case of zero intermediate frequency (zero-IF, also known as direct conversion architecture), two ADCs are used, one for the in-phase component and one for the quadrature component. For example, FIG. 1 is a block diagram of a known communication apparatus having an antenna array 110 comprising several receive antennas 120 and several receivers 140. The receive antennas 120 are also known as antenna array elements or antenna elements. The receive antennas 120 provide respective analog receive signals 122 that are filtered by respective bandpass filters 130. Each receiver 140 includes an analog to digital converter (ADC) (not shown in FIG. 1). A digital signal processor (DSP) 180 performs processing associated with MIMO spatial multiplexing and beamforming.
Having several receivers and ADCs results in higher cost and power. The cost and power increase proportionally with the number of antenna elements in the array 110. Thus, using arrays that have more than a few receive antennas may be impracticable due to the cost and amount of power required. Therefore, it can be seen that there is a need for an architecture that can provide greater throughput at a lower cost.